Pre-marital sex is a penetrative vaginal or anal sexual intercourse performed before formal marriage in younger population. Moreover, these population groups are less likely than adults to have the information, skills and support to protect themselves against HIV/AIDS and other consequences of premarital sex. The aim of this study was to assess the prevalence of pre-marital sexual practice and its associated factors among high school students in Addis Zemen town. A cross sectional study design was conducted in purposefully selected site from April to June 2017. Single population proportion formula was used to select 284 study subjects. Study participants were selected randomly from grade 9 and 10 students. Structured pre-tested questionnaire was used to collect the data and data were checked and entered to EpiInfo v.3.5. Statistical Packages for Social Sciences v.20 used to analyze the data. Bivariant analysis results with p-value <0.2 was included in multivariate logistic regression. The study showed a prevalence of 32.6% premarital sex. Being lower age groups (13-18) were 2.48 times at higher risk than those who are at high age groups (19-25), (AOR=2.48, CI= 1.24-4.95). Having girl/boyfriends were 20.66 times at higher risk than those who have no girl/boyfriend, (AOR=20.66, CI= 9.39-45.46). Premarital sexual practice among the study participants was strongly associated with being lower age groups and having boy/girlfriends. The most mentioned reason for students to start their first sexual practice was love affair and interest. Health education should be encouraged and the clinic at school level should be strengthened as well as establishing a club to teach about premarital sexual practices are suggested.